Vaterite in freshwater cultured pearls from China and Japan

Japanese and Chinese beaded and non-beaded cultured freshwater pearls of good quality were investigated with Raman spectroscopy and LA-ICP-MS. In 50% of the investigated non-beaded samples vaterite was identified by Raman spectrometry in polished cross-sections near the centre of the pearl as well as in small blemishes on the pearl surfaces. Continuous growth structures transect both vaterite and aragonite areas. Sodium and strontium concentrations are significantly lower in vaterite areas whereas the Mg concentration is up to two orders of magnitude higher and ratios of these elements allow distinction of the two phases using LA-ICP-MS. With the findings that vaterite is relatively common in the centres of high-quality freshwater cultured pearls and is the reason for lack of orient on some pearl surfaces, it is necessary to fully understand how it forms in order to optimize the quality of such pearls.